Load-carrying skeletal members, such as the human hip, frequently are rendered non-functional because of fracture, damage, disease, resections for malignancy or disease or because of pain or malformation. Such members are commonly repaired by total joint replacements with artificial components. One type of bone replacement that has been particularly successful over the past thirty years is that of the human hip. Such hip prostheses typically include a femoral portion or component which is implanted in the femur and an acetabular component which is secured to the pelvis. The femoral component includes a head which rotates in a socket formed in the acetabular component. A collar is often provided on the femoral component which rests on a surface on the proximal femur.
Many known hip prostheses require the use of cement for installation of the femoral component into the medullary canal of the femur. One type of cement which is commonly used is methyl methacrylate.
Success of the femoral component of a total hip implant depends in large part on the technical precision with which the implant is inserted. There are several factors which contribute to the success of a femoral component. First, for a cemented component, the component should be centered within the central cavity in the medullary canal of the femur into which the femoral component is inserted. Centering of the component insures that the thickness of the cement mantle surrounding the component is uniform on all sides. Uniformity of the cement mantle renders the load distribution at the bone-cement and metal-cement interfaces generally uniform on all sides of the component, thus avoiding problems associated with overstressing one area of the interface, such as fracturing of the mantle or separation of the mantle from the bone or separation of the component from the mantle. Centering of the component can be particularly difficult for those components which do not include a collar.
Another factor which has been identified as contributing to the success of either an uncemented or a cemented femoral component is that the femoral component should be rotated about its axis into the proper angular position with respect to the femur for stability and range of motion. Proper rotational position, or so-called anteversion, allows for accurate reproduction of the mechanical orientation of the hip joint.
A third factor is that the component should be prevented from rotating once it is seated in the femur. For cemented components, such rotational control is very important, particularly during insertion and hardening of the cement, and any false motion while the cement is hardening has been found to be detrimental to the overall results of a cemented femoral stem. Uncontrolled rotation prior to hardening of the cement could result in a stem which is not properly centered and which does not have the proper angular position once the cement hardens. For uncemented components, it is still important that rotational stability be achieved after implantation of the component.
To reduce manufacturing costs and inventory requirements, it is desirable to standardize components to the greatest extent possible, so that one style or design can be used for most patients. Since different sizes of components are required for patients of different stature or age, the manufacture and storage of different styles for each size component is considered highly undesirable. However, the strength, configuration and amount of available bone on the proximal femur varies greatly from patient to patient, even for patients who require the same size components. For example, on many patients the bone mass on the proximal femur is so small or is configured such that only a small portion of the collar on the femoral component rests on a bone surface. Thus, standardization requires that the design selected for a component be able to accommodate these large differences in strength, configuration and size.
Many efforts have been made in the past to design components which resist rotation or which tend to be self centering. Examples of such components include those found in the following U.S. Pat. Nos.: 5,116,380; 5,108,452; 4,946,379; 4,936,863; 4,783,192; 4,770,660; 4,678,571; 4,623,353; 4,535,487; 4,068,324; 4,012,796; 2,719,522; and 2,682,265. However, none of the foregoing designs is completely successful in both preventing rotation of the component once implanted, and insuring that the component is held in a properly centered position. In addition, some of the foregoing designs would not operate to prevent rotation or lateral movement in all femurs due to the limited lateral extent of the devices used. In some patients, the devices would not engage any bone because of its irregular configuration or lack of bone mass. Moreover, while spacers, such as those disclosed in U.S. Pat. No. 5,116,380, have been used for the purpose of automatically centering the component within the medullary canal, such spacers do not serve to prevent rotational movement of the prosthesis during cement hardening. Finally, spacers can interfere with the movement of the cement around the edges of the component, thus, on occasion producing voids or gaps in the cement mantle.
It is therefore an object of the present invention to provide an improved femoral component for a hip prosthesis.
It is another object of the present invention to provide a femoral component which is self centering.
It is a further object of the present invention to provide a femoral component which allows the physician to insert and maintain the component with the proper angular position.
It is another further object of the present invention to provide a femoral component which is prevented from rotating or moving laterally during hardening of the cement, and which can be used with many different sizes and shapes of bones.
It is yet another further object of the present invention to provide a method and apparatus for inserting into a femur an improved femoral component.
It is also yet another further object of the present invention to provide a device for centering of a femoral component which does not have a collar.